Attention modulates P1 component onset

Research Report Cognitieve Neurowetenschappen

K. Bangel, B. Hilhorst, K. Jagersberg, D.Portain, A. Siebold,

4/8/2009

ABSTRACT

Posner et al proposed a three-factor model of attention addressing different

cognitive and neural correlates. Following a study from Luck et al (1998), we

confirmed the link between attention and P1 amplitude for P1 onset latency as

well. In the current study, ERP recordings of six participants executing a variant

of the Posner paradigm are investigated. P1 latency was analyzed for segments

containing trials with attention paid to the left versus attention paid to right

visual field. For each of both sides, slow versus the fast responses were

compared, as obtained from individual reaction times. The results yield that P1

component is strongest on the side contralateral to the attended visual field

compared to the ipsilateral side, suggesting that attention has a mediating effect

on the P1 component. Attention might act as a mechanism that increases neural

sensitivity, leading to an increased activity during stimulus detection.

Additionally, attention might optimize neural pathways to fasten the response

upon stimulus detection. When comparing fast RT opposed to slow RT, small

differences in P1 onset latency suggest that P1 serves as an indicator of RT

which can be observed before the actual response.

Key Terms: Attentional modulation, Spatial Cueing Task, ERP, P1.

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INTRODUCTION

The shifting and focus of attention by humans is a topic that has been

examined in depth within the cognitive science. Different mechanisms have been

identified, such as the Simon-effect (Simon, 1969) or change blindness

(Henderson and Hollingworth., 1999). Posner and Peterson's 3-factor model of

attention suggests three general attentional functions incorporated by distinct

regions of the brain: Orienting to sensory events (primarily processed in

posterior parietal cortical areas and subcortical areas), detection of target

stimuli (located in an anterior attention system) and alerting (frontal attentional

systems on the right hemisphere) (Posner & Peterson, 1990).

Attentional processes can be examined by means of event-related potentials

(ERPs). Research on attention commonly utilizes ERPs based on recorded EEG

data. This technique offers the possibility to observe ERP waveforms that are

associated with attentional processes. One of the earliest components of the

evoked response to a visual stimulus is the P1 component and is visible in the

ERP waveform. The P1 component typically occurs 80 to 140 ms after stimulus

onset and is strongest in the Parieto-Occipital areas PO7 and PO8. Luck, Heinze,

Mangun and Hillyard (1989) lined out that the P1 component can be understood

as reflecting a facilitation of sensory processing of items at an attended location

and can therefore serve as a measure for the presence and absence of attention.

It their experiment, differences in amplitude were observed in the P1 component

for attended stimuli and unattended stimuli when subjects performed a stimulus

detection task.

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It is possible that not only the amplitude of the P1 component can be

facilitated by attention but its latency as well. In that case, visual attention

directed at one side of the visual field may result in an earlier onset of the P1

component for attended stimuli compared to unattended stimuli.

The task used in the study by Luck et al. (1989) required the subject to

perform a visual search for a target stimulus from a group of stimuli. As we are

interested in latency differences of the P1 component we use a task that omits

this search and only requires stimulus detection, the Posner task.

In this experiment subjects perform an altered version of the Posner spatial

cueing task paradigm. Subjects are asked to attend to either the left of the right

side of a computer screen as indicated by a visual cue. After a short interval two

stimuli appear, one on each side of the screen. The subjects have to respond to

the orientation of the stimulus (which is either horizontal or vertical) on the

attended side with a corresponding key press. Since we are interested in

attention influencing the P1 component, the task does not involve invalid cueing,

unlike the original Posner task. Stimuli will be presented bilaterally in each trial.

As the stimuli will be presented in different visual fields, each stimulus will be

processed in the corresponding hemisphere, hence each trail two different P1

components will be measures (unattended and attended).

Both reaction times and brain activity are measured by a computer and EEG

measurement requipment. ERPs will be constructed using the EEG data and will

be examined on their amplitude and latency. We expect to observe amplitude

differences in the P1 component of attended and unattended stimuli in

conformity with the study by Luck et al. (1989). We also expect to see a latency

difference of the P1 component onset as well for attended or unattended stimuli.

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Finally we also observe behavioral data (response times) in order to examine if

the Posner task may be subject to a Simon effect as the mapping of the response

keys in relation to the target stimulus can be either congruent or incongruent. If

this effect is present, it may influence the P1 component since it is attention-

related. We expect the Simon effect to be present, possibly to a lesser degree as

attention is directed by the cue.

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METHODS

PARTICIPANTS

EEG recordings were made from two female and four male students aged

between 21 and 28 (mean 23.8), following a practical EEG recording course at

the University of Twente. All participants were right handed, not under actual

medical treatment and had no history of psychiatric or neurological disorders.

All participants had normal or corrected to normal vision, and accurate hearing

abilities. One student did not have intact color vision and two participants were

excluded from analysis due to procedural errors. All participants gave informed

consent and the experiment was approved by the local ethics committee.

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Figure 1. Example of the stimuli and their temporal order as employed in the

experiment.

Stimuli, apparatus and recording procedure

All stimuli were presented on a black 17’’ computer screen. During each trial

a small white fixation circle was continuously presented in the centre of the

screen, attended by two white circles located at 12.2° to the left and right of the

fixation dot. After 700ms showing the default display the fixation dot changed

into a bigger dot for 400ms, again the default display appeared for 600ms after

which the cue (two opposing red and green triangles, each pointing to one of the

circles) replaced the fixation point for 400ms. Next, the default display was

presented again for another 600ms followed by the target, presented in one of

the circles (see Figure 1). Targets consisted either of a horizontal or a vertical

black line appearing for 200ms within one of the white circles. The default

display was shown again for 1800 ms after target onset. Participants were

seated in front of a screen at a distance of about 70 cm in a silenced and partly

darkened chamber. The software Presentation (version 11.0) was used for

stimulus presentation.

DATA ACQUISITION

EEG data were recorded using 12 Ag/AgCl ring electrodes placed on a

standard 10/10 cap. The channels were recorded at the specific locations: (F3 Fz

F4 C3 Cz C4 P3 Pz P4 PO7 Oz PO8). Electrode impedance was held below 5 kΩ.

Button triggers, EEG en EOG data were amplified by a Quick-Amp

(BrainProducts GmbH) and recorded at a sample rate of 1000 Hz with

BrainVisionRecorder (Version 1.4). EOG were measured above and below the

left eye, and horizontally on the outer canthi of both eyes to determine the vEOG

and the hEOG.

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TASK AND PROCEDURE

Participants were instructed to keep their eyes on the fixation dot while

performing 320 trials, divided into four blocks. At the beginning of the first two

blocks, participants were instructed to pay attention to the direction of the

green arrow, pressing the left CTRL key whenever the indicated circle was filled

with a vertical line and pressing the right CTRL button when the indicated circle

was filled with a horizontal line. Following 20 practice trails, they completed two

blocks of each 80 randomized experimental trials varying in color (green vs. red)

and direction (left vs. right) of arrow cue and type (horizontal vs. vertical) and

location of target cues. Responses with the corresponding hand were regarded

as correct response, whereas responses with the non-corresponding hand were

regarded as false response.

Prior to the start of the third and forth block instructions were chanced.

Participants were then asked to pay attention to the direction indicated by the

red arrow.

DATA ANALYSIS

Behavioral data was analyzed by one-way-ANOVA. RT were splitted into two

conditions: “congruent” and “incongruent”. To account for the Simon effect, the

congruent condition was specified as directing attention to the same spatial side

as the desirable button response. Accordingly, trials including button responses

after attending to contrary side were specified as incongruent conditions.

EEG data were digitally filtered (TC = 0 s, high-cutoff filter of 200 Hz, notch

filter of 50 Hz) by Brainvision Recorder. Using the median of all RT for each

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participant, our data covered by the time window of interest between -100 and

250ms from stimulus onset were distinguished into a slow and a fast condition.

We determined the baseline from -100 to 0 ms before the stimulus was

presented.

Reactions which occurred 100ms after target presentation were regarded

premature and rejected. Also rejected were segments showing false responses.

Segments were then removed from EOG artifacts by rejecting horizontal EOG

amplitudes greater than 60µV and vertical EOG amplitudes greater than 120µV.

Subsequently, EEG artifacts were removed by rejecting segments with

amplitudes greater than 100µV. To determine which side of the screen was

attended, we distinguished the remaining EEG data into two halves representing

attentive processes on either the left or the right target.

After creating a grand average over all subjects, ERP data was filtered

through a lowpass of 16 Hz to further enhance the signal-to-noise-ratio. After

selecting the most promising electrodes (PO7 and PO8) for further analysis, we

compared the segments containing trials with attention paid to the left versus

attention paid to the right screen side. For each of both lateral sides, we

compared the slow versus the fast condition.

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RESULTS

If participants showed more than 5% incorrect trials on basis of their

behavioral data, they were excluded from further analysis. This criterion applied

for one participant, leaving 4 participants with usable data for further analysis.

EEG rejection criteria ultimately lead to the exclusion of 14.4% from all trials.

BEHAVIORAL DATA

The analysis of reaction times did not yield any significant results. In

particular there were no significant differences between trials in congruent and

incongruent conditions (F<1; p>0.05).

EEG DATA

LATERAL COMPARISON

Regarding both sides of the scalp, we observed a difference in P1 latency

between conditions in which attention was paid to the left side versus attention

was paid to the right side. In both cases, activation of the contralateral electrode

showed a significant shorter latency (by about 8 ms) than the activation of

ipsilateral electrode, as can be seen in Figure 2. The difference in amplitudes

regarding P1 components was insignificant.

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Figure 2. Comparison of P1 components in the “attend left” and “attend right”

condition for the electrodes P7 (top) and P8 (bottom)

RESPONSE SPEED COMPARISON

Only results regarding the right visual field yielded a significant shorter

onset delay for the P1 component in the fast condition. The difference in

amplitudes indicated a weaker response for the slow condition, compared to the

fast condition (Figure 3 and Figure 4). These effects were significant for both

hemispheres.

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Figure 3. Comparison of P1 component for fast (straight line) vs. slow

responses (dotted line) in the “attend left” condition for the

electrodes P7 (top) and P8 (bottom)

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Figure 4. Comparison of P1 component for fast (straight line) vs. slow

responses (dotted line) in the “attend right” condition for the

electrodes P7 (top) and P8 (bottom)

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DISCUSSION

The results obtained in this experiment suggest that attention has a

mediating effect on the P1 component when subjects perform the Posner task.

This mediating effect manifests in two ways: when comparing the P1 component

on locations PO7 and PO8, we can see that the amplitude of the P1 component is

strongest on the side contralateral to the visual field that the subject is

attending to. It may be possible that attention acts as a mechanism that

increases sensitivity for a stimulus for a specific region of the brain, in this case

either PO7 or PO8. This increased sensitivity may lead to an increased neural

response when a stimulus is detected, with more neurons firing after stimulus

detection resulting in increased amplitude of the P1 component. Second, the

faster onset of the P1 component for attended stimuli versus unattended stimuli

may suggest that attention even serves as a mechanism that optimizes neural

pathways in order to fasten the response upon stimulus detection.

When comparing the P1 components of fast reaction times opposed to slow

reaction times for either visual field, there are some small differences in the

onset of the P1 component. This suggests that before the actual response the P1

component serves as an indicator of the reaction time of the response. Only did

this effect occur for the condition that subjects were attending to the right visual

field. This can be explained from research on hemispatial neglect patients,

indicating an attentional bias for the left visual field by default (eg. Kinsbourne,

1987), resulting in less noticeable differences between the P1 components of

fast or slow response times.

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Analysis of the response times did not provide evidence that responses were

faster when a subject was presented a congruent stimulus compared to

incongruent stimuli. As such there is no Simon-effect (Simon & Rudell, 1967)

present in the Posner task, most likely due to the presence of a cue that directs

attention to the relevant visual field before the stimulus is shown.

Some considerations have to be made when drawing conclusions from the

results of this experiment. First, the experiment only featured the data of four

test subjects out of a total of six. Certain effects may be more apparent when

there are more subjects participating in the experiment. One of those effects is

the LRP, which was not clearly visible for both the stimulus-locked and

response-locked waveforms. The same is true for the analysis of the P1

component for the fast and slow responses when subjects are attending the left

visual field.

Second, for analyzing the P1 components for fast and slow responses, all

responses were divided into either “fast” responses (all responses faster then

the median response time) or “slow” responses (all responses slower then the

median response time). A different technique features the division of the

response times into three groups rather than two and using the fastest group as

“fast” responses and the slowest group as “slow” responses. Such a division may

have yielded more pronounced effects.

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FUTURE RESEARCH

Following from this study there are some recommendations regarding

possible future research possibilities. As has been stated before, there are

indicators that the P1 component can be influenced by the attentional state of

the subject. First, a replication study featuring can be conducted featuring more

subjects. With more subjects it is possible to test the effects for significance and

draw solid conclusions regarding the indicators of this study.

Another topic for future research is to explore to which extent the

preparation interval can influence the P1 component. It seems likely that some

preparation is required on the neural level when a subject directs his or her

attention to a particular visual field and that this preparation takes some time.

In this study the time delay between the cue and the stimulus was constant so

the preparation time was the same for every trial. It is, however, possible that

the length of the preparation interval determines the degree of neural

preparation and that adjusting the length of this interval may influence the

onset and amplitude of the P1 component.

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REFERENCES

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Kinsbourne, M. (1987). Mechanisms of unilateral neglect. In M. Jeannerod (Ed.),

Neurophysiological and neuropsychological aspects of spatial neglect. Amsterdam:

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dissociation of P1 and N1 components. Electroencephalography and Clinical

Neurophysiology, 75, 528–542.

Posner M. I., Peterson S. E. (1990). The attention system of the human brain.

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Simon, J. R. (1969). Reaction toward the source of stimulation. Journal of Experimental

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Simon, J. R. & Rudell, A. P. (1967). Auditory S-R compatibility: the effect of an irrelevant

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